Flexographic inks are solvent based inks applied by rollers or pads to flexible sheets or rolls of plastic foil and paper. The practical aspects of the use of these ink resins, and inks derived therefrom, require that the polyamide resin be soluble in alcoholic solvents and such solubility be obtained without sacrificing toughness, adhesion and gloss.
Environmental concern over the amounts of volatile organic solvents in the atmosphere has led to a desire to use aqueous solutions that have less volatile organic solvents contained therein. In order to accommodate the reduced levels of volatile organic solvents, the polyamide resins used as binders should have increased water solubility and yet retain the other desirable properties of polyamide resins such as those based on the polymeric fatty acids.
U.S. Pat. Nos. 3,253,940 and 3,224,893 illustrate the polyamide resins of polymeric fatty acids used in the past in formulations utilizing alcoholic solvents particularly ethanol in which varnishes of the polyamides in alcohol solvents were on the order of 35% by weight nonvolatile solids.
The foregoing patents provided resins which could be employed with the usual alcoholic solvents. However, as environmental solutions were sought, efforts were made to reduce emissions such as those from the volatile alcohol solvents. One means of reducing the emission was to provide water-reducible polymeric fatty acid polyamides as illustrated in U.S. Pat. No. 3,776,865. As disclosed therein, this was achieved by acid termination of the polymeric fatty acid polyamides employing an acid component of the polymeric fatty acid and another codicarboxylic acid and an amine component comprising isophorone diamine alone or in admixture with conventional diamines such as the alkylene diamines i.e., ethylene diamine. Acid termination was achieved by employing about 50-75 amine equivalents per 100 carboxyl equivalents. Varnishes of these resins in an alcohol solvent such as n-propanol on the order of about 40% non-volatile solids are disclosed.
Another U.S. patent, U.S. Pat. No. Re. 28,533 dealing with polymeric fatty acid polyamides employing lower aliphatic mono-basic acids such as acetic and propionic with certain amine combinations disclosed a few solubilities in ethanol up to 60% though many were 50% or below.
As environmental standards have become more and more stringent, effort have continued to provide resins which comply with such standards. High solids varnishes on the order of 55-60%, and preferably above 60%, which are formulated into pigmented inks will meet the solvent emissions standards desired in order to reduce solvent emissions. U.S. Pat. No. 4,508,868 discloses polyamides prepared from polymeric fatty acids and diamines which include in the acid component an unsaturated fatty acid monomer and which employ relative amine and carboxyl amounts so as to provide an acid terminated product having an acid value in the range of 8-20 and preferably in the range of 10-15. Such products could be employed in alcoholic ink varnishes at levels of 60% solids or higher.
However, the polyamide resins from polymeric fatty acids, while providing acceptable properties in most instances, do not possess the desired water solubility or dispersibility when employed in inks or varnishes. Accordingly attempts have been made to improve the hardness and water solubility products which contain even lower volatile organic solvents. Commonly assigned co-pending application Ser. No. 701,903 filed Feb. 15, 1985 by Whyzmuzis, et al, now abandoned, was an attempt to overcome the deficiencies of the polymeric fatty acid polyamide resins. This patent application discloses polyamides which are substantially free from polymeric fatty acids which have improved water solubility in their use with flexographic or gravure ink binders. While such resins provided generally acceptable products, there was still room for improvement in the hardness or non-tacky properties of the resins.